The disclosure of Japanese Patent Application No. 2000-151301 filed on May 23, 2000, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a method and apparatus to control a continuously variable transmission of a motor vehicle, and more particularly to the control of the continuously variable transmission during an operation of an anti-lock brake system.
2. Description of Related Art
Motor vehicles having a continuously variable transmission (CVT), a shift control device, and an anti-lock brake system (ABS) are known. The continuously variable transmission is disposed in a power transmission path between a drive power source for running the vehicle and drive wheels. This type of transmission is capable of continuously varying the speed ratio (i.e., input-shaft rotational speed/output-shaft rotational speed), which is the ratio of the speed of rotation of an input shaft on the power source side to the speed of rotation of an output shaft on the drive wheel side. The above-indicated shift control device is operable to control the speed ratio of the continuously variable transmission in accordance with predetermined shift conditions. The anti-lock brake system (ABS) is operable to control the braking force so as to avoid locking of the drive wheels during braking. An example of such vehicles is disclosed in Japanese laid-open Patent Publication No. 6-156121. In the vehicle disclosed in this publication, the shift control is performed during the operation of the ABS, based on an estimated vehicle body speed, instead of the vehicle speed that corresponds to the wheel speed, or the speed of rotation of the output shaft, so that a vehicle operator or driver is less likely to feel bumping of the vehicle (or shift shocks) that would be caused by fluctuations in the speed ratio of the continuously variable transmission due to changes in the wheel speed caused by the ABS.
However, since the estimated vehicle body speed is calculated by the ABS, the number of communication lines between the ABS and the control apparatus of the CVT is increased, and the control apparatus, which employs a CPU, or the like, having the function of communicating with the ABS, tends to be complicated and be available at an increased cost.
Even though the above arrangement can reduce or suppress fluctuations in the speed ratio of the continuously variable transmission during the operation of the ABS, there is still a problem of slipping of a belt of the belt-and-pulley type CVT in which the power is transmitted by use of frictional force. More specifically, as the speed of rotation of the output shaft (or wheel speed) changes, the inertia torque of a power source for running the vehicle acts on the continuously variable transmission, thus causing slipping of the transmission belt of the CVT. In view of this problems, it has been proposed, for example in Japanese laid-open Patent Publication No. 11-44359, to increase a belt clamping force or pressure for an increase in the frictional force between the belt and pulleys in order to prevent the aforementioned belt slippage. However, in the known continuously variable transmission in which shift control and clamping force control are performed by using hydraulic oil, sufficiently large belt clamping force may not be generated when the hydraulic oil is consumed for a shifting operation of the continuously variable transmission. In the case where a mechanical oil pump driven by the power source for running the vehicle is used to generate a hydraulic pressure, for example, if the speed of rotation of the pump and the amount of oil delivered from the pump are reduced with a reduction in the vehicle speed, a sufficiently large amount of the hydraulic oil and sufficiently large belt clamping force may not be provided. In order to avoid such shortage of the hydraulic oil, it may be considerable to reduce the rate of change in the speed ratio of the continuously variable transmission. In this case, however, the speed ratio may not be returned to a predetermined ratio at which the vehicle is to be stopped, which ratio is normally equal to the maximum speed ratio, before the vehicle stops.
It is therefore an object of the invention to provide a method and an apparatus for controlling a continuously variable transmission, which reduces fluctuations in the speed ratio of the transmission during an operation of an ABS, thereby to avoid bumping of the vehicle, without using an estimated vehicle body speed, while further reliably avoiding slipping of a transmission belt due to variations in the inertia torque during the operation of the ABS.
To accomplish the above and/or other objects, one aspect of the invention provides a method and an apparatus for controlling a continuously variable transmission of a motor vehicle that is provided in a power transmission path between a power source that runs the vehicle and at least one drive wheel, in which a speed ratio of the transmission, which is a ratio of a rotational speed of an input shaft on the side of the power source to a rotational speed of an output shaft on the side of the drive wheel(s), can be continuously varied. The method and apparatus include a controller that: (a) controls the speed ratio of the continuously variable transmission in accordance with a predetermined shift condition, and (b) smoothes at least one of the rotational speed of the input shaft and the rotational speed of the output shaft for use in control of the speed ratio, during an operation of an anti-lock brake system for controlling braking force to prevent the drive wheel from locking during a braking operation.
In the method and apparatus constructed as described above, fluctuations in the speed ratio of the continuously variable transmission due to increases and decreases in the output-shaft rotational speed during the operation of the ABS are suppressed or reduced, so that the vehicle is less likely to suffer from bumping (shift shocks) due to the changes in the speed ratio. Furthermore, since at least one of the input-shaft speed NIN and the output-shaft speed NOUT are smoothed for the shift control, the control apparatus can be easily constructed at a reduced cost, as compared with the case where shift control is performed based on an estimated vehicle body speed that is determined by the ABS, or the like.